The ATF believes they know what your intent is. That's all they need. You have parts, well, it's your intent to build a naughty thing. You have to understand, the ATF has this ridiculous idea that they're above the law. They believe that they know what is best for you and society. If you are lucky, when they suspect you of something, you are notified. If they suspect you of something that they deem heinous, you will be notified around 2AM and your dog will most likely be a victim.
They should be focusing on smuggling in and out of the country. Coordinate with homeland security against foreign and domestic threats. Bonafide threats, not some jilted ex-lovers revenge plot.
We as Americans should be allowed suppressors, machine pistols , fully automatic whatever if we so choose with the caveat of a background check which we have now for firearm purchases. It shouldn't matter if I own a 3 round burst drum and safety or not. It shouldn't matter if I have an extra hole in my receiver or a drop-in sear, just as it shouldn't matter if I have a folding stock or a short barrel this or that. At the end of the day, It's our right to own, laid out in our bill of rights, it's our responsibility to not give up those rights.
I don't like alcohol, I don't preach prohibition to reduce drunk driving fatalities. What's the difference? Each right entails responsibility. The ATF is an over-reaching agency that should be abolished or admonished to doing its job and its job alone. You can't legislate by loose interpretation, it doesn't work that way.